An organic light emitting diode is a light emitting device using an organic electroluminescence (hereinafter, referred to as “organic EL”), and generally, has a configuration in which an anode and a cathode are provided on both surfaces of an organic EL layer that includes a light emitting layer containing an organic light emitting material. As the organic EL layer, an electron transport layer, a hole transport layer, and the like are provided as necessary in addition to the light emitting layer. Examples of an organic light emitting diode include a bottom emission type in which an anode formed from a transparent conductive material such as ITO, an organic EL layer including a light emitting layer, and a cathode formed from a metal are sequentially formed on a transparent substrate such as a glass substrate, and light is extracted from a substrate side, a top emission type in which the cathode, the organic EL layer, and the anode are sequentially formed on the substrate, and light is extracted from a side opposite to the substrate side, and the like.
The organic light emitting diode has advantages in that the viewing angle dependency is small, the power consumption is low, and the thickness thereof is very small. On the other hand, the organic light emitting diode has a problem in that the light extraction efficiency is low. The light extraction efficiency represents a ratio of energy of light emitted to the air from a light extraction surface (for example, in a case of a bottom emission type, a substrate surface) to energy of light emitted from the light emitting layer. For example, light emitted from the light emitting layer is output in all directions, and thus the light enters into a waveguide mode in which the majority thereof repeats total reflection on an interface between a plurality of layers having refractive indexes different from each other. As a result, the light is converted into heat while propagating between layers, or emitted from a side surface, and thus the light extraction efficiency decreases. In addition, since the light emitting layer is close to the cathode formed from a metal, a part of near-field light from the light emitting layer is converted into a surface plasmon on a surface of the cathode, and disappears. As a result, the light extraction efficiency decreases.
The light extraction efficiency has an effect on brightness of a display, an illuminating device, and the like that are provided with the organic light emitting diode, and thus various methods has been reviewed to improve the light extraction efficiency. As one of the methods of improving the light extraction efficiency, a method of using surface plasmon resonance is suggested. For example, PTL 1 to PTL 4 disclose a method of providing a one-directional or two-directional periodic microstructure on a surface of a metallic layer (cathode). In this method, the periodic microstructure functions as a diffraction lattice. Due to this, energy, which disappears as the surface plasmon on the surface of the cathode is extracted as light, and thus the light extraction efficiency is improved.